In general, a window (opening) of a building is a location for the transfer in and out of a lot of heat. For example, a rate of heat loss flowing from a window in a case of heating in winter is approximately 48% and a rate of heat inflowing from a window in a case of cooling in summer reaches even approximately 71%. Therefore, it is possible to obtain an effect of enormous energy saving by appropriately controlling light and/or heat through a window.
An optical control glass has been developed for such a purpose and has a function of controlling an inflow and/or outflow of light and/or heat.
There are some kinds of methods for executing an optical control of such an optical control glass, and it is possible to provide, for example, the following materials:
1) an electrochromic material with an optical transmittance that is reversibly changed by applying an electric current and/or an electric voltage thereto;
2) a thermochromic material with an optical transmittance that is changed depending on a temperature; and
3) a gasochromic material with an optical transmittance that is changed by a control of an atmospheric gas.
Among these, a study of an electrochromic optical control glass that uses a tungsten oxide thin film for an optical control layer is most advanced, so that a stage of practical application has generally been attained at present and a marketed product has also been provided.
However, an electrochromic optical control glass wherein a tungsten thin film is used for this optical control layer is such that a principle thereof is that light is absorbed by the optical control layer to execute an optical control. Therefore, there is a problem in that the energy saving effect is degraded because an optical control layer absorbs light so as to be heated and it is also re-radiated into the room interior. In order to eliminate this, an optical control is not executed by absorbing light and it is necessary to execute an optical control by reflecting light. That is, a material (reflection-type optical control element) has been desired that has a characteristic in such a manner that a state thereof is reversibly changed between a transparent state and a reflective state.
For a material that has such a characteristic, it has been found and reported in recent years that a state is reversibly changed between a transparent state and a reflective state due to hydrogenation and dehydrogenation of a rare earth metal such as a yttrium or a lanthanum (see, for example, Patent Document 1).
Otherwise, an alloy of a rare earth metal such as gadolinium and magnesium (see, for example, Patent Document 2), an alloy of magnesium and a transition metal (for example, see Patent Document 3), and an alloy of an alkaline-earth metal such as calcium and magnesium (for example, Patent Document 4) have already been known as a material that has a reflection-type optical control characteristic (optical control mirror characteristic).
However, there is a problem of degradation in the above-mentioned reflection-type optical control element in such a manner that switching between a transparent state and a reflective state is hardly executed after the switching is repeated. It is found that this major cause is that magnesium in a magnesium alloy layer being an optical control layer penetrates a palladium layer being a catalyst layer and emerges on a surface thereof, with repetition of switching, and such emerging magnesium is oxidized to be magnesium oxide so that hydrogen is not allowed to penetrate therethrough.
For this reason, a reflection-type optical control element with a thin film of a metal such as titanium, niobium, or vanadium as a buffer layer being inserted between an optical control layer and a catalyst layer has been developed in order to prevent such magnesium from emerging on a surface (see, for example, Patent Document 5).
However, even when a buffer layer is provided, about 500 times of switching causes degradation, and as it is exceeded, switching is hardly executed. Furthermore, there is a problem in that a transmittance of visible light in a transparent state is reduced by insertion of a buffer layer (see, for example, Patent Document 5).
Although fabrication of an element that is not degraded even when switching is repeated 10000 or more times is successful by using Mg1-x-yYxScy for an optical control layer (see, for example, Patent Document 6), a film thickness of an expensive palladium catalyst layer is increased for an optical control element with a high durability in order to inhibit permeation of oxygen into an optical control layer. Moreover, there is also a problem in that a transmittance in a transparent state is decreased as a film thickness of the catalyst layer is increased.